NCP1060, NCV1060, NCP1063, NCV1063
www.onsemi.com
18
Fault Condition – Output Short−circuit
As soon as V
CC
reaches V
CC(on)
, drive pulses are
internally enabled. If everything is correct, the auxiliary
winding increases the voltage on the V
CC
pin as the output
voltage rises. During the start−sequence, the controller
smoothly ramps up the peak drain current to maximum
setting, i.e. I
IPK
, which is reached after a typical period of
4 ms. When the output voltage is not regulated, the current
coming through COMP pin is below I
COMPfault
level (40
m
A
typically), which is not only during the startup period but
also anytime an overload occurs, an internal error flag is
asserted, Ipflag, indicating that the system has reached its
maximum current limit set point. The assertion of this flag
triggers a fault counter t
SCP
(48 ms typically). If at counter
completion, I
pflag
remains asserted, all driving pulses are
stopped and the part stays off in t
recovery
duration (about
400 ms). A new attempt to re−start occurs and will last
48 ms providing the fault is still present. If the fault still
affects the output, a safe burst mode is entered, affected by
a low duty−cycle operation (11%). When the fault
disappears, the power supply quickly resumes operation.
Figure 32 depicts this particular mode:
Figure 32. In case of short−circuit or overload, the NCP106X protects itself and the power supply via a low
frequency burst mode. The V
CC
is maintained by the current source and self−supplies the controller.
IpFlag
Timer
DRV
internal
48 ms typ.
400 ms typ.
Fault
Open loop FB
V
CC(on)
V
CC(min)
V
CC
V
COMP
Auto−recovery Over Voltage Protection
The particular NCP106X arrangement offers a simple
way to prevent output voltage runaway when the
optocoupler fails. As Figure 33 shows, a comparator
monitors the V
CC
pin. If the auxiliary pushes too much
voltage into the C
VCC
capacitor, then the controller
considers an OVP situation and stops the internal drivers.
When an OVP occurs, all switching pulses are permanently
disabled. After t
recovery
delay, it resumes the internal drivers.
If the failure symptom still exists, e.g. feedback
opto−coupler fails, the device keeps the auto−recovery OVP
mode. It is recommended insertion of a resistor (
R
limit
)
between the auxiliary dc level and the V
CC
pin to protect the
IC against high voltage spikes, which can damage the IC,
and to filter out the Vcc line to avoid undesired OVP
activation. R
limit
should be carefully selected to avoid
triggering the OVP as we discussed, but also to avoid
disturbing the V
CC
in low / light load conditions.
Self−supplying controllers in extremely low standby
applications often puzzles the designer. Actually, if a SMPS
operated at nominal load can deliver an auxiliary voltage of
an arbitrary 16 V (V
nom
), this voltage can drop below 10 V
(V
stby
) when entering standby. This is because the
recurrence of the switching pulses expands so much that the
low frequency re−fueling rate of the V
CC
capacitor is not
enough to keep a proper auxiliary voltage.
